In optical disc technologies, data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity and detecting the light that has been modulated by, and reflected from, the optical disc. On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by evaporation process, for example, on the surface of a substrate on which spiral lands or grooves are arranged. In writing data on a rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits and tracks and the thickness of the recording material film are both smaller than the thickness of the optical disc substrate. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called a “storage plane” or an “information plane”. However, considering that such a plane actually has a physical dimension in the depth direction, too, the term “storage plane (or information plane)” will be replaced herein by another term “information storage layer”. Every optical disc has at least one such information storage layer. Optionally, a single information storage layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
In a recordable or rewritable optical disc, when data is going to be written on its information storage layer, the information storage layer is irradiated with such a light beam, of which the optical power has been modulated as described above, thereby recording an amorphous mark on a crystalline phase change material layer. Such an amorphous mark is recorded there by heating a portion of the information storage layer that has been irradiated with a writing light beam to a temperature that is equal to or higher than its melting point and then rapidly cooling that portion. If the optical power of a light beam that irradiates the recorded mark is set to be relatively low, the temperature of the recorded mark being irradiated with the light beam does not exceed its melting point but the recorded mark will turn crystalline again after having been cooled rapidly (i.e., the recorded mark will be erased). In this manner, the recorded mark can be rewritten over and over again. However, if the optical power of the light beam for writing data (i.e., optical recording power) had an inappropriate level, then the recorded mark would have a deformed shape and sometimes it could be difficult to read the data as intended.
Such an amorphous recorded mark has a different reflectance from its surrounding crystalline portions. For that reason, when a read operation is performed, the intensity of the reflected light varies depending on whether or not a recorded mark is there. In an area where data has already been written (which will be referred to herein as a “recorded area”), there is a series of recorded marks and spaces, of which the lengths are variable with the contents of the data to be written. For that reason, the optical properties (i.e., the optical reflectance and transmittance) of such a recorded area are different from those of an area where no data has been written yet (which will be referred to herein as an “unrecorded area”).
To read data that is stored on an optical disc or to write data on a rewritable optical disc, the light beam always needs to maintain a predetermined converging state on a target track on an information storage layer. For that purpose, a “focus control” and a “tracking control” need to be done. The “focus control” means controlling the position of an objective lens along a normal to the surface of the information plane (such a direction will sometimes be referred to herein as “substrate depth direction”) so that the focal point (or at least the converging point) of the light beam is always located on the information storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as a “disc radial direction”) so that the light beam spot is always located right on a target track.
In order to perform such a focus control or a tracking control, the focus error or the tracking error needs to be detected based on the light that has been reflected from the optical disc and the position of the light beam spot needs to be adjusted so as to reduce the error as much as possible. The magnitudes of the focus error and the tracking error are represented by a “focus error (FE) signal” and a “tracking error (TE) signal”, both of which are generated based on the light that has been reflected from the optical disc.
Dual-layer optical discs, in which two information storage layers are stacked one upon the other, have already been put on the market recently. And now, so-called “multilayer optical discs”, including a stack of three or more information storage layers, are also being developed. In the following description, however, an optical disc in which N layers (where N is an integer that is equal to or greater than two) are stacked one upon the other (i.e., any optical disc with at least two layers) will be referred to herein as a “multilayer optical disc”.
When data is being read from, or written on, a target one of the information storage layers of a multilayer optical disc, the optical disc drive needs to set the focus position of the light beam on the target information storage layer and form a tiny light beam spot on that information storage layer. As a single multilayer optical disc has multiple information storage layers, if the focus position of a light beam is set on the deepest information storage layer, for example, that light beam should pass all of the other information storage layers that are shallower than the deepest layer.
Unless the intensity of a light beam (i.e., the optical recording power) is optimized when data is going to be written, a recorded mark will be deformed as described above, and therefore, the read error rate will rise. Thus, in order to optimize the optical recording power, data are sometimes tentatively written on a test write area of an information storage layer of an optical disc with the optical recording power changed into multiple different values and the data thus written are read. In this manner, a read error index may be set on the test write area and an optical recording power associated with the best index may be selected. Strictly speaking, however, the optical recording power optimized in this manner is nothing but “initial optical recording power”. That is to say, after data has started to be written on a user data area with the initial optical recording power thus determined, the initial optical recording power will be corrected as needed into a more appropriate level based on a β value to be described later, for example. Such processing to be performed by an optical disc drive on the test write area in order to determine the initial optical recording power will be referred to herein as “optimum power control (OPC)”.
FIG. 1(a) illustrates a multilayer optical disc 10 and FIG. 1(b) is a schematic cross-sectional view thereof. The multilayer optical disc 10 shown in FIG. 1 includes a first information storage layer L0, which is located deepest under the disc surface 10a on which a light beam is incident, and a second information storage layer L1, which is located closest to the disc surface 10a. The multilayer optical disc 10 has a user data area on which user data will be written and a test write area, which will also be referred to herein as a “power calibration area (PCA)” and which is located inside of the user data area. Although the multilayer optical disc 10 actually has other management areas in addition to the PCA, those areas are not shown in FIG. 1 for the sake of simplicity.
FIG. 2 illustrates in further detail a portion of the cross section of the multilayer optical disc 10 shown in FIG. 1(b). In FIG. 2, illustrated schematically are three light beams that are respectively focused on three different areas a, b and c on the information storage layer L0. Specifically, the area a is a part of the PCA on which no data has been written yet on any of the two information storage layers L0 and L1 (i.e., an unrecorded area). Likewise, the area b is a part of the user data area on which no data has been written yet on any of the two information storage layers L0 and L1 (i.e., an unrecorded area). Meanwhile, the area c is a part of the user data area on which data has been written on the information storage layer L1 (i.e., a recorded area).
Portions of the information storage layer L1 on which data has been written have a different optical transmittance from the rest of the same layer L1 on which no data has been written yet. In the majority of optical discs, a recorded area has a lower optical transmittance than an unrecorded area. Thus, the light beam focused on the area c of the information storage layer L0 has been transmitted through a portion of the information storage layer L1 that has a decreased optical transmittance. As a result, the intensity of the light beam on the information storage layer L0 is lower in the area c than in the area b.
In this manner, the quantity of light that the light beam focused on the information storage layer L0 can give to that information storage layer L0 changes depending on whether or not data has been written on the information storage layer L1 that is located shallower (i.e., closer to the disc surface) than the information storage layer L0.
FIG. 3 is a graph showing how the rate of errors caused by reading data from the information storage layer L0 changes with the optical recording power. In FIG. 3, the abscissa represents the optical recording power (which will be sometimes simply referred to herein as “power”) and the ordinate represents the error rate during reading (which is the error rate of the L0 layer). Specifically, one curve shown in FIG. 3 indicates results that are obtained in the areas a and b, which are unrecorded areas, and the other curve shown in FIG. 3 indicates results that are obtained in the area c, which is a recorded area.
As can be seen from FIG. 3, in the areas a and b, the error rate is the lowest when the optical recording power is PWb. In other words, it can be seen that the optical recording power is preferably set to be PWb in the unrecorded areas a and b. In the area c, on the other hand, the error rate is the lowest when the optical recording power is PWc. That is to say, it can be seen that the optical recording power is preferably set to be PWc, which is greater than PWb, in the recorded area c. Consequently, it is preferred that data be written on the recorded area c with higher optical recording power than on the unrecorded areas a and b.
According to currently available techniques, when data is going to be written on a target location with a particular address in the user data area of a given optical disc, it is not clear whether that location falls within a recorded area or an unrecorded area. That is why the initial optical recording power to start writing data with is set to be the best value for the unrecorded area (e.g., PWb in the example illustrated in FIG. 3). In that case, after data has started to be written on the user data area, the optical recording power is corrected at short intervals by reference to an index indicating a read signal waveform while reading the data.
FIG. 4(a) illustrates a cross section of the multilayer optical disc 10 just like FIG. 2. FIGS. 4(b) and 4(c) show how the optical recording power needs to be, or need not be, corrected with time in two different situations where data has started to be written on the area b, which is an unrecorded area, and on the area c, which is a recorded area, with the initial optical recording power PWb in both cases.
In the example illustrated in FIG. 4(b), the optical recording power is maintained at PWb, which is the optimum value for the area b. On the other hand, in the example illustrated in FIG. 4(c), the optical recording power is corrected so as to increase toward the optimum value PWc for the area c.
The greater the number of information storage layers included in a multilayer optical disc, the more and more often there are two or more information storage layers that are located shallower than the deepest information storage layer L0 when a light beam is focused on that layer L0. For that reason, in the recorded areas on those shallower information storage layers, the optical transmittance will decrease more and more significantly.
FIG. 5(a) schematically illustrates a cross section of a multilayer optical disc with three information storage layers L0, L1 and L2. FIGS. 5(b) and 5(c) show how the optical recording power needs to be, or need not be, corrected with time in two different situations where data has started to be written on the area b, which is an unrecorded area, and on the area e, which is a recorded area, with the initial optical recording power PWb in both cases.
In the example illustrated in FIG. 5(c), the optical recording power is corrected so as to increase toward the optimum value PWe for the area e. However, it might take a longer time for the optical recording power to reach that optimum value PWe than in FIG. 4(c). The reason is that as data has already been written in the area e on both of the information storage layers L1 and L2, the optical transmittance would decrease significantly in that area and the optimum optical recording power value PWe for the area e could be quite different from the optimum optical recording power PWb for the areas a and b.
The number of information storage layers included in a multilayer optical disc should continue to increase from now on. And most multilayer optical discs could have four or more information storage layers in the near future. Even so, the same problem should arise in those multilayer optical discs. That is to say, if data started to be written with the optical recording power PWb that has been optimized for an unrecorded area a, the optical recording power could not be corrected into a more appropriate value in a reasonably short time. Thus, to overcome such a problem, Patent Document No. 1 discloses a technique for performing an OPC on the PCA for every possible combination of recorded and unrecorded areas.
Besides adopting the technique disclosed in Patent Document No. 1, Patent Document No. 2 also teaches managing the addresses of the recorded and unrecorded areas so as to determine whether the target location falls within a recorded area or an unrecorded area and then changing the initial optical recording power based on a result of that decision.
CITATION LIST
Patent Literature
                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2008-108388        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2008-192258        